Wilting Tomatoes on a Brackish Well
Mrs Charles grows tomatoes on 2 hectares of coastal land in southern Jamaica. For three seasons she has irrigated from a well that is becoming brackish. This season she also spread 400 kg of NPK fertiliser per hectare - twice the recommended rate - three days before a week of heavy rain. Now plants in the seaward half of the field wilt sharply at midday and recover only partly overnight, while plants in the inland half look normal. A soil analysis reports that the water in the seaward soil contains dissolved salts at an equivalent concentration of about 0.18 mol/dm³, compared with 0.03 mol/dm³ inland. The cell sap of root hair cells in both areas is equivalent to about 0.10 mol/dm³. A neighbouring farmer says the wilt is 'root rot' and offers to sell her a fungicide. She has J$18,000 set aside for inputs and asks you, the school's agriculture club adviser, whether to spend it on the fungicide.
1. Explain, in terms of water potential, why the seaward plants wilt while the inland plants do not. Use the figures given in the case.
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Water moves by osmosis from a region of higher (less negative) water potential to one of lower (more negative) water potential. Dissolved solute makes a solution's water potential more negative, so the seaward soil solution - equivalent to 0.18 mol/dm³ - has a more negative water potential than the cell sap of the root hair cells, which is equivalent to only 0.10 mol/dm³. Water therefore leaves the root hair cells and enters the soil instead of being taken up. The inland soil (0.03 mol/dm³) is less concentrated than the cell sap, so water continues to enter those roots and the plants stay turgid.
The fertiliser and the rain make it worse: the rain dissolves the fertiliser and surface runoff carries the dissolved salts downslope into the seaward half, where they accumulate as water evaporates from the surface. At midday, rapid transpiration lowers the water potential in the leaves and pulls on the water column, so the loss of turgor shows as wilting; when stomata close in the afternoon, water loss slows and the plants partly rehydrate overnight.
2. Evaluate the neighbour's advice. Would you spend the J$18,000 on fungicide? Give two alternative actions and the reasoning behind each.
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No, the money should not go on fungicide. The damage follows a gradient: the whole seaward half is affected and the inland half is normal, which matches the salt figures rather than the patchy, plant-to-plant spread typical of a soil-borne pathogen. There are no reported lesions, root rot or damping-off, and the partial overnight recovery is a water-relations effect - infected roots would not recover and re-wilt on that daily cycle. A fungicide cannot change the water potential of the soil solution, so it would not remove the cause, and the J$18,000 would be wasted.
Two better uses of the money:
- Change the water source - harvest rainwater or use a fresher source, applied by drip irrigation - so the soil solution becomes less concentrated than the root cell sap (well below 0.10 mol/dm³ equivalent) and water enters the roots again.
- Cut the fertiliser to the recommended rate and split the applications, and mulch the beds. Less solute enters the soil, and mulch reduces the surface evaporation that concentrates salts around the roots. A salt-tolerant variety for the seaward block is worth considering if a fresh water supply cannot be secured.
3. Before spending any money, Mrs Charles wants a quick, cheap test to confirm the cause. Describe what she should do and what result would support the salinity explanation.
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Test the soil solution rather than the plants. Take several soil samples from each half, mix each sample with the same volume of distilled water (for example 1 part soil to 2 parts water), stir, allow to settle and filter. Then either measure the electrical conductivity of the filtrate with a cheap EC meter, or run a simple living test: stand equal pieces of peeled potato in the two filtrates and in distilled water as a control, or sow equal numbers of quick-germinating seeds such as lettuce or bean, and compare mass change, germination and root length after two to three days.
If the seaward filtrate has the higher conductivity, and the potato pieces lose mass or the seeds germinate poorly in it while doing well in the inland filtrate and in distilled water, the salinity explanation is supported. A field check is also possible: irrigate a one-metre patch in the seaward block with about 20 litres of rainwater and see whether those plants stop wilting within a few days. If they do, water potential is the problem and the fungicide money is better spent on water.